PURPOSE: To provide a receiving mouth-working mold having good durability and maintenance properties and capable of working even a small caliber pipe material.

CONSTITUTION: A plurality of first air cylinders 21... and a plurality of second air cylinders 22... being actuators moving the other end parts 16... of divided pieces 13a, 13b in a radius direction are alternately radially arranged to the base end part 12c of a mold main body 12 and engaging pins 23a are engaged with engaging holes 17. Therefore, the reciprocating movement of pistons 24 is transmitted to the inside divided pieces 13b through piston rods 23 to revolve the inside divided pieces 13b in the radius direction centering around one end part 15.